1. Field of the Invention
The present invention relates to a monolithic piezoelectric actuator of multilayer design which has a high aspect ratio of more than two and which is built up from a plurality of smaller stacks in a multilayer design.
Piezoelectric actuators normally include a plurality of piezoelectric elements arranged in a stack. Each of these elements, in turn, includes a piezoceramic layer which is provided on both sides with metallic electrodes. If a voltage is applied to these electrodes, the piezoceramic layer reacts with a lattice distortion which leads to a usable lengthwise expansion along a major axis. Since this amounts to less than two parts per thousand of the layer thickness along the major axis, a correspondingly higher layer thickness of active piezoceramic must be provided in order to achieve a desired absolute lengthwise expansion. With increasing layer thickness of the piezoceramic layer within one piezoelectric element, however, the voltage necessary for the response of the piezoelectric element also rises. In order to keep the voltage within manageable limits, the thicknesses of individual piezoelectric elements normally lie between 20 and 200 .mu.m. A piezoelectric actuator must therefore have an appropriate number of individual elements or layers for a desired lengthwise expansion.
Known piezoelectric actuators of multilayer design therefore summarily include numerous individual layers. For their production, piezoceramic green films are alternatively arranged with electrode material to form a stack. The layers are then laminated and sintered together to form a monolithic composite. Such a process is known, for example, from an article by S. Takahashi et al. in Ferroelectrics, 1983, Vol. 90, pages 181 to 190. Larger actuators with larger absolute deflection are obtained, for example, by adhesively bonding a plurality of such stacks. Such a process is disclosed, for example, by U.S. Pat. No. 5,438,242. However, such bonded stacks have too low a stiffness for many applications, in particular if high forces have to be transmitted using the piezoelectric actuator. Sufficiently high stiffnesses are possessed only by piezoelectric actuators of fully monolithic multilayer design. Only the latter exhibit a sufficiently solid composite of the individual layers in the stack.
In the production of monolithic actuators of multilayer design, however, additional problems occur with increasing height. The plates which are laminated to form a stack and which contain numerous individual actuators must be divided up before sintering. Here, the stacks of the relatively large-area green films are divided up into smaller stacks with the desired actuator area. While low stacks can be stamped in automatic machines, such as is possible, for example in the case of multilayer capacitors, in the case of higher stacks this must be replaced by a multiple sawing process along the separating lines.
In the case of low stacks, the lamination can take place in automatic machines with short cycle times. Higher stacks must be laminated with increased care in order, in particular, to maintain the vertical structural accuracy during lamination. In the process there is always the risk that, as a result of the applied pressure, transverse flow processes in the green films will lead to a displacement of individual layers in relation to one another. In the zones with which contact is to be made later, the accuracy of contact is thereby destroyed.
The organic binders which are used in multilayer technology are primarily matched to the requirements of the green film production and also lamination. They also must be removed before sintering by means of a complicated binder removal process in furnaces with a monitored atmosphere. However, the diffusion paths for the binder or its decomposition products during the binder removal process are multiplied with increasing stack height. In order to prevent destruction of the stack as a result of too high an internal pressure of the decomposing binder, it is necessary to take very elaborate technical measures, much more sophisticated than those in the case of the production of multilayer capacitors.
Because of the many problems to be solved in the production of piezoelectric actuators of multilayer design, known monolithic piezoelectric actuators in the cost-effective technology which is normal in the manufacture of multilayer capacitors were until now restricted to a maximum height of about 2.5 to 5 mm. In addition, for the reasons mentioned, these known piezoelectric actuators only achieve an aspect ratio (height/width) of a maximum of about 2. Higher actuators and actuators with a higher aspect ratio were previously obtained by adhesively bonding together a plurality of smaller stacks; the stiffness of the stack and hence its mechanical loadability being reduced.
It is therefore an object of the present invention to provide a simple and reliable production process for a piezoelectric actuator of monolithic multilayer design which possesses a high aspect ratio of more than 2, which can be produced monolithically in stack heights of more than 5 mm and which possesses optimum piezoelectric properties and a high mechanical composite strength.